Fix import error when running examples in fresh environment (#19431)

(cherry picked from commit 5aea3b1)

requirements/pytorch/examples.txt

@@ -1,6 +1,7 @@
 # NOTE: the upper bound for the package version is only set for CI stability, and it is dropped while installing this package
 #  in case you want to preserve/enforce restrictions on the latest compatible version, add "strict" as an in-line comment
 
+requests <2.32.0
 torchvision >=0.14.0, <0.17.0
 gym[classic_control] >=0.17.0, <0.27.0
 ipython[all] <8.15.0

src/lightning/pytorch/demos/transformer.py

@@ -9,16 +9,19 @@
 from pathlib import Path
 from typing import Dict, List, Optional, Tuple
 
-import requests
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
+from lightning_utilities.core.imports import RequirementCache
 from torch import Tensor
 from torch.nn.modules import MultiheadAttention
 from torch.utils.data import DataLoader, Dataset
 
 from lightning.pytorch import LightningModule
 
+_REQUESTS_AVAILABLE = RequirementCache("requests")
+
+
 if hasattr(MultiheadAttention, "_reset_parameters") and not hasattr(MultiheadAttention, "reset_parameters"):
     # See https://github.com/pytorch/pytorch/issues/107909
     MultiheadAttention.reset_parameters = MultiheadAttention._reset_parameters
@@ -125,6 +128,11 @@ def __getitem__(self, index: int) -> Tuple[Tensor, Tensor]:
 
     @staticmethod
     def download(destination: Path) -> None:
+        if not _REQUESTS_AVAILABLE:
+            raise ModuleNotFoundError(str(_REQUESTS_AVAILABLE))
+
+        import requests
+
         os.makedirs(destination.parent, exist_ok=True)
         url = "https://raw.githubusercontent.com/pytorch/examples/main/word_language_model/data/wikitext-2/train.txt"
         if os.path.exists(destination):